1 Technical Field of the Invention
The present invention relates generally to a fabrication machine for a spark plug which may be employed in automotive engines and a spark plug fabrication method using the same, and more particularly to such a spark plug fabrication machine capable of bending a plurality of ground electrodes of a spark plug to an L-shape uniformly and a spark plug fabrication method using the same.
2 Background Art
FIG. 1 shows a typical two-ground electrode spark plug 1 which consists of a metal housing or shell 4 on which ground electrodes 3 and a central electrode 2 are installed, a porcelain insulator 5, a metal stem 7 with a terminal 6, and a resistor 8 having a given electric resistance. Electric sparks are produced between the ground electrodes 3 and the central electrode 2 to ignite a gaseous fuel.
The fabrication of each of the ground electrodes 3 is achieved by welding a bar electrode to an end of the metal shell 4 and bending it to an L-shape. This type of spark plug, however, has a difficulty in bending two ground electrodes to a uniform shape for reasons as discussed below.
FIG. 7 demonstrates a typical method of bending straight bar electrodes 3a after welded on the metal shell 4 to fabricate the ground electrodes 3. First, a die 13 having curves contoured to conform with the L-shape to which the bar electrodes 3a are to be finished is interposed between the bar electrodes 3a. Press bars 14 are moved from the outsides of the die 13 in approach directions to push the bar electrodes 3a against the curves of the die 3, thereby bending the bar electrodes 3a to the L-shape. The metal shell 4 and the die 13 are held firmly by, for example, a die chuck.
The die 13 is usually easy to form to have the curves conforming in contour with a desired L-shape of the ground electrodes 3. It is, however, difficult to weld the bar electrodes 3a to the metal shell 4 at desired locations accurately. This may result in variations in distances d1 and d2, as shown in FIG. 8(a), between the bar electrodes 3a and the center of the metal shell 4 which are usually greater than a dimensional error of the die 13. Specifically, different gaps xcex41 and xcex42, as shown in FIG. 8(b), are created between inner walls of the bar electrodes 3a and side walls of the die 13, thereby, as shown in FIG. 8(c), resulting in differences in radius of curvature between the die 13 and the bar electrodes 3a after bent, which leads to a difference in contour between the ground electrodes 3. This will cause a undesirable difference in springback between the ground electrodes 3 to be produced, so that the distance between one of the ground electrodes 3 and the center electrode 2 will differ from that between the other ground electrode 3 and the center electrode 2, thereby resulting in instability of sparks.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide a spark plug fabricating machine capable of bending ground electrodes to be uniform in shape and a spark plug fabricating method using the same.
According to one aspect of the invention, there is provided a fabricating method capable of bending ground electrodes of a spark plug to a substantially L-shape uniformly. The ground electrodes have ends facing through spark gaps a center electrode installed in a housing. The spark plug fabricating method comprises the steps of: (a) welding at least two electrode bars to the housing of the spark plug at a given interval away from each other; (b) placing a first die at a location where the center electrode is to be installed between the electrode bars, the die having curves each contoured to conform with the L-shape of the ground electrodes; (c) moving a second die to urge one of the electrode bars into contact of an inner surface thereof with the first die; (d) pushing the second die to press the one of the electrode bars against a corresponding one of the curves of the first die, thereby bending the one of the electrode bars to fabricate one of the ground electrodes of the L-shape; (e) moving a third die to urge the other electrode bar into contact of an inner surface thereof with the first die; and (f) pushing the third die to press the other electrode bar against the other curve of the first die, thereby bending the other electrode bar to fabricate the other ground electrode of the L-shape.
In the preferred mode of the invention, the fabricating method further comprises the step of holding the housing so as to allow each of the electrode bars welded on the housing to move relative to a corresponding one of the second and third dies for absorbing a gap between each of the electrode bar and the first die. The holding step also places the housing so that each of the electrode bars is located at a given interval away from one of deformation avoiding blocks installed outside the electrode bars.
The interval is between 0.2 mm and 0.25 mm.
Each of the second and third dies is implemented by a rotary roller having a peripheral surface which urges a corresponding one of the electrode bars into engagement with the first die.
Each of the rotary rollers has formed in the peripheral surface thereof a groove in which a corresponding one of the electrode bars is engaged when bent.
The fabricating method may further comprise the step of, after each of the electrode bars is bent, adjusting the spark gap between the electrode bar and the center electrode.
Each of the pushing steps may bend one of the electrode bars to an angle greater than a bend angle of the ground electrodes for compensating for springback.
According to the second aspect of the invention, there is provided a spark plug fabricating machine designed to fabricate at least two L-shaped ground electrodes to have ends facing through spark gaps a center electrode extending along a longitudinal center line of a spark plug. The spark plug fabricating machine comprises: (a) a housing holder holding a housing of the spark plug to which at least two electrode bars are joined at a given interval away from each other; (b) a first die held between the electrode bars, the first die having curves each contoured to conform with an L-shape of the ground electrodes; (c) a press mechanism moving a second die to press each of the electrode bars against a corresponding one of the curves of the first die, thereby bending the electrode bar to a desired angle for fabricating each of the L-shaped ground electrodes; and (d) a holder supporting mechanism supporting the housing holder so as to allow each of the electrode bars joined to the housing to swing in substantially the same direction as movement of the second die by the press mechanism for absorbing a clearance between each of the electrode bars and the first die to establish a direct contact between the electrode bar and the curve of the first die when the electrode bar is bent by the second die.
In the preferred mode of the invention, die blocks are further provided which have faces opposed to the electrode bars through a given gap, respectively, for avoiding unwanted bulging of the electrode bars toward the second die when pressed by the second die.
The gap is preferably between 0.2 mm and 0.25 mm.
The second die includes rotary rollers each having a peripheral surface which urges a corresponding one of the electrode bars into engagement with the first die.
Each of the rotary rollers has formed in the peripheral surface thereof a groove in which a corresponding one of the electrode bars is engaged when bent.
A hammer may be provided which works to strike tips of the electrode bars after bent by the second die for adjusting a gap between each of the ground electrodes and the center electrode.
The press mechanism may bend each of the electrode bars to an angle greater than a bend angle of the ground electrodes for compensating for springback.